feat: add Nix language support

[uxtechie]

Summary

Add full Nix (.nix) language support to CodeGraph, including symbol extraction, import resolution, and export detection.

Closes #332

Related: #324 — While that issue requests distributing CodeGraph as a Nix flake (packaging), this PR adds support for parsing and indexing Nix source files as a new language in the extraction engine.

What this PR does

Symbol Extraction (src/extraction/languages/nix.ts)

• Functions: Extracts named function bindings with parameter signatures, including:
  • Simple parameters: y = arg: arg + 1; → signature (arg)
  • Destructured parameters: z = { name }: ... → signature { name }
  • Curried functions: f = a: b: c: body → signature a : b : c
  • Destructuring with @ alias: { x, y } @ args and args @ { x, y } patterns
• Variables: Extracts non-function bindings as variable nodes
• Inherit declarations: Processes inherit and inherit (expr) as variable nodes
• Call references: Extracts function calls from apply_expression, distinguishing import expressions (emitted as imports references) from regular calls
• Export detection: Implements isExportedNode() helper that traverses AST ancestors to determine if a symbol is in the top-level attribute set (exported) vs. inside a let binding or nested attribute set (private)

Import Resolution

• src/resolution/import-resolver.ts: Registers .nix and /default.nix extensions in EXTENSION_RESOLUTION
• src/resolution/index.ts: Adds file-level import resolution for imports-kind references — resolves directly to file: node IDs instead of symbol name matching. Also adjusts the pre-filter to avoid discarding valid import references

Supporting Files

• src/extraction/grammars.ts: Registers tree-sitter-nix grammar (vendored WASM, loaded from src/extraction/wasm/ like Pascal, Scala, Lua, and Luau)
• src/extraction/languages/index.ts: Registers Nix extractor
• src/types.ts: Adds nix to the Language type
• src/extraction/wasm/tree-sitter-nix.wasm: Bundled WASM parser (no new runtime or dev dependencies added — the grammar was compiled offline and vendored directly)

Test Coverage

Extraction tests (__tests__/extraction.test.ts)

• Basic variable and function extraction
• inherit and inherit (expr) handling
• Import expression extraction (relative paths and builtins.import)
• Function call reference extraction
• Destructured and curried function signatures
• Export/scope detection (top-level exported vs. let-local vs. nested private)

Resolution tests (__tests__/resolution.test.ts)

• Relative Nix import path resolution to file nodes (import ../core/ports.nix)

Verification

• npm run build compiles cleanly with no type errors
• All 745 tests pass (29 test files), including all new Nix-specific tests
• No regressions in any other language extractor

Dependencies

No new runtime or dev dependencies. The tree-sitter-nix WASM grammar is vendored in src/extraction/wasm/ (same approach used for Pascal, Scala, Lua, and Luau). The package.json dependencies and devDependencies sections are unchanged from main.

Files Changed

File	Change
src/extraction/languages/nix.ts	New — Full Nix extractor
src/extraction/wasm/tree-sitter-nix.wasm	New — Vendored WASM grammar
src/extraction/grammars.ts	Register nix grammar
src/extraction/languages/index.ts	Register nix extractor
src/types.ts	Add nix to Language type
src/resolution/import-resolver.ts	Add .nix extensions
src/resolution/index.ts	File-level import resolution
__tests__/extraction.test.ts	Nix extraction test suite
__tests__/resolution.test.ts	Nix import resolution test